1. Technical Field of the Invention
The invention relates to a method and apparatus for generating digital image data. More particularly, this invention relates to digital image generation with, for example, a digital copying machine or a free-standing scanner apparatus.
In apparatus of this kind, a document having an image thereon is scanned with an electro-optical converter to generate digital image data which contain a grey value for each image element or pixel. This is usually done with a CCD array, on which a part of the document in the form of a line is projected by an optical imaging system. By moving either the document or the imaging system in a direction perpendicular to the scanning line of the document, the document is scanned completely, line-by-line, in accordance with a usually rectangular raster of pixels.
The spatial density or resolution of the pixels is, in practice, on the order of some hundreds of pixels per inch, usually specified as "dpi" (dots per inch).
2. Description of Related Art
Digital image data of the kind described above can be processed in the spatial domain, but it has also been proposed to process such data in the frequency domain. The latter domain is for instance particularly suited for filtering operations, in which disturbing elements, which often have frequencies outside the range of those of the image information, can be removed easily from the digitised image.
In this connection, also the combined use of image data generated from one image by two scanners each having a resolution different from the other, is proposed.
For instance, U.S. Pat. No. 5,121,445 describes a method aimed at removing moire patterns. Patterns of this kind occur when a rastered image is scanned with a scanner, due to the fact that the raster frequency of the image interferes with the scanning frequency (resolution) of the scanner. The interference causes frequency domain sidebands on either side of the spectrum of the original non-rastered image. Also, the entire spectrum, including the sidebands, is periodically repeated as a result of the discrete scanning by the scanner. These sidebands contain information on both the rastering and the scanning. The known method comprises eliminating the side bands as far as possible by comparing the spectra of the image data from the two scanners, so that the original non-rastered image can be reconstructed. Thus, this method not only removes the moire but also the raster. Effectively, therefore, information present in the scanned image is destroyed.